RevoluteJoint applying torque to chassis

The problem I have is that when I apply torque to the two revolute joints, it applies some torque to the chassis as well. I can see this clearly if I drop the car into free-fall, and then apply torque to the revolute joints. The chassis then starts rotating.

I would like to have the chassis not have any torque applied when I apply torque to the revolute joints. Is this a bug in Farseer Physics?